Generally, in any cooling arrangement that employs adiabatic material for effecting the evaporative cooling of air, the adiabatic material or evaporative pads are arranged substantially vertical. Moisture, usually water, is deposited on an upper portion of the evaporative pads and by the force of gravity, the moisture gradually descends across the evaporative pads giving the water a chance to be absorbed into the evaporative material. For proper wetting of the evaporative pads, a sufficient amount of moisture is required to saturate the evaporative material in order to achieve the maximum evaporative cooling effect possible that results from forcing air through the saturated evaporative material. As air is forced through the adiabatic material, it is cooled by the action of evaporation. As this occurs, the amount of moisture absorbed into the adiabatic material reduces as the moisture in liquid form is vaporized hence cooling the air passing through the adiabatic material by extracting thermal energy from same in order to vaporize the water. Any water that is not absorbed into the evaporative pads is called run-off and this is primarily pumped back to the top of the evaporative pads to save water.
There are two methods commonly used to deposit water over the top of the evaporative pads. The first method employs a water recirculation system that is designed to pump run-off water from a sump and recirculate it up to the evaporative pads. As water is evaporated, the decreased water level in the sump is sensed and the evaporated water is replaced by adding make-up water to either the sump or directly to the top of the evaporative pads as is known in the art. The amount of make-up water necessary is usually monitored by a sump float switch. The float switch is set to ensure a minimum amount (height) of water exists which is required to maintain a positive head of water pressure to allow proper pump operation.
The second method used to deposit water over the top of the evaporative pads is called a “once-through” system so termed because the water that is deposited to the top of the evaporative pads is not recirculated but it travels only once through the system. In the once-through system, fresh water is deposited to the top of the evaporative pads by energizing a make-up solenoid valve any time the pads are desired to be wetted. In the once-through system, any excess water not absorbed by the pads goes down the drain. While the once-through system is simpler and less expensive in initial cost, it uses more water.